The present invention relates to a focusing device for use in optical apparatuses such as microfilm readers or microfilm reader-printers for enlarging and projecting images of various types of microfilms such as roll type or fiche type to read or print the projected image.
In optical system in the microfilm readers or the microfilm reader-printers for enlarging and projecting images of a microfilm, high magnifications are required. Accordingly, the microfilm readers are so constructed that a distance between a microfilm and a projective lens is very short as compared with the distance between the projective lens and an image surface. Under such construction, a little fluctuation in the distance between the microfilm and the projective lens results in a seriously blurred image because of the shift of focus.
Nevertheless, conventional microfilm readers or microfilm reader-printers employ a manual focusing operation method which is difficult and consumes much time to achieve the in-focus condition. Even once the in-focus condition has been achieved, the condition is likely to be lost or shifted by vibration or by operator's slight touching or hitching on the device, and so forth. Such a shift will raise particularly serious problems when printing is made by projecting an image of a microfilm on a photoreceptor. Consequently, it is necessary to manually make focusing operation and to correct the operation again and again when being out of focus. Thus, the operators sustain inconvenience.
On the other hand, Japanese Patent Laid-open Nos. 60-227241 and 61-247157 (laid open on Nov. 12, 1985 and on Nov. 4, 1986 respectively) disclose known devices provided with automatic focusing function which enables automatic attainment of the in-focus condition for microfilm readers.
Such Japanese conventional devices include a sensor for receiving light located at the side of an image surface for automatic focusing achieved by means of the fact that the sensor receives light coming through a projective lens to detect condition of an image formed.
This method is convenient, because the focusing is automatically performed. However, the known sensor in the conventional devices is positioned out of the screen on which the image to be read is projected through an ordinal optical path. With this arrangement, it is necessary for the film to be provided with a special mark or image so as to project the image thereof onto the sensor for detecting the focus condition. Another known sensor in the conventional devices is positioned in the other optical path which is divided or changed from the ordinal optical path by deflection of a mirror. With this arrangement, the body of the microfilm reader will be increased in size because the part of the projection light temporarily deviating from the ordinal optical path must be accommodated in the body which is mostly occupied by the optical path. A mechanism for temporarily deflecting the projection light is further necessary, resulting in complex structure and high cost.